OBD CodesWhat it means
Many modern diesels spray a urea solution, sold as diesel exhaust fluid or AdBlue, into the exhaust. In a special catalyst it turns harmful nitrogen oxides into nitrogen and water. These codes mean the system is not cutting emissions enough, the fluid is low or poor, or a sensor has failed.
Codes in this group
| Code | Meaning |
|---|---|
| P203F | Diesel exhaust fluid level too low |
| P204F | Diesel exhaust fluid system not performing as expected, bank 1 |
| P20E8 | Diesel exhaust fluid pressure too low |
| P20EE | SCR catalyst efficiency below limit, bank 1 |
| P2200 | NOx sensor circuit fault, bank 1 |
| P2201 | NOx sensor reading out of range, bank 1 |
| P2202 | NOx sensor signal low, bank 1 |
| P2203 | NOx sensor signal high, bank 1 |
| P2204 | NOx sensor signal cuts in and out, bank 1 |
Bank 1 is the side of the engine that has cylinder 1. Sensor 1 is before the catalytic converter and sensor 2 is after it. More terms explained.
What you may notice
- Warning message about emissions or exhaust fluid
- A countdown of distance left before the engine will not restart, on many cars
- Usually no change in how the car drives at first
Common causes
Most likely first:
- Fluid level low, or fluid that is old, diluted or contaminated
- A blocked or crystallised dosing injector
- A failed pump or heater in the fluid tank
- A faulty NOx sensor
- A worn SCR catalyst
What to check first
- Top up with fresh fluid that meets ISO 22241, from a sealed container
- Never put the fluid in the diesel tank, or diesel in the fluid tank
- Do not ignore a restart countdown: once it reaches zero the car may refuse to start
Have a mechanic confirm. A fault code tells you which system reported a problem, not which part has failed. The same code can have several causes, and they differ between car models. Use this page to understand the code, then have a qualified mechanic test the car before any part is replaced.
After the repair
Once the cause is fixed, the code can be cleared with a scan tool, or the warning light goes out by itself after several trips without the fault. Clearing a code without fixing the cause only hides it for a while. More about clearing codes.
Related codes
Exhaust gas recirculation (EGR) (P0400), Secondary air injection (P0410), Catalytic converter efficiency (P0420), Fuel vapour (EVAP) system (P0440), Diesel particulate filter (DPF) (P2002). All codes.
Written in November 2026. These are generic OBD-II codes, the same on all makes. Causes and repair steps differ between models.